Fix parsing of migration item name for binNMU in tpu

The parsing of migration items should also look for the suite name in the
architecture part. This fixes the parsing for migration items like
some-src/amd64_tpu and some-src/amd64_tpu/1.0-1

Signed-off-by: Ivo De Decker <ivodd@debian.org>

britney2/migrationitem.py

@@ -97,6 +97,9 @@ class MigrationItem(object):
             else:
                 self._architecture = 'source'
 
+        if '_' in self._architecture:
+            self._architecture, suite_name = self._architecture.split('_', 2)
+
         if self._version in self.__class__.get_architectures():
             (self._architecture, self._version) = \
             (self._version, self._architecture)